A network is composed of electric/electronic devices which are connected using a data bus and between which data is exchanged via the bus. The electric/electronic devices are referred to as network nodes, or as nodes for short. If there is precisely one transmission path between two respective nodes in a network, the network is referred to as an acyclical network.
There are different network standards, depending on the data bus used and the electric/electronic devices connected to the data bus. A network standard to which a lot of attention has been paid recently is the standard IEEE 1394. The standard IEEE 1394 provides a standard which is suitable in particular for supporting the exchange of data between multi-media devices of any desired type. A data bus which is configured according to the IEEE 1394 standard can be connected both to a personal computer and its peripherals such as printers, scanners, CD-RW drives and hard disks and to home entertainment electronic devices such as video cameras or televisions.
In a network bus according to the IEEE 1394 standard or the standard IEEE 1394a, all the nodes must be able to transmit data at the speed S100, which corresponds to a transmission speed of approximately 100 Mbps. A node can optionally also support the speed S200 or the speeds S200 and S400. The maximum transmission speed for exchanged data between the individual nodes at the level of the bit transmission layer, the so-called PHY transmission speed, is determined here by the speeds of so-called physical layer chips (PHY chips) of the nodes involved in the physical exchange of data. In addition, each node with the so-called link-layer chip (link chip), which is arranged in the protocol hierarchy of the IEEE 1394 network above the PHY chip has a further speed determining component. The PHY chip and the link chip of a node can have different maximum speeds which are referred to below as the PHY speed or link speed of the node. The link chip of a node is, however, involved only in the transmission of data if the respective node itself is a starting node or destination node of a transmission of data. If a node merely passes on a transmission of data because it lies on the transmission link between the starting node and the destination node or nodes, only its PHY chip is involved in the transmission of data. The PHY transmission speed of a transmission of data between a starting node and one or more destination nodes of the network bus is equal to the minimum value of the PHY speeds of the nodes lying on the transmission link, including the starting node and the destination node or the destination nodes. The maximum actual transmission speed of a transmission of data is equal to the minimum value of the link speeds of the starting node and the destination node or the destination nodes and the PHY transmission speed of the transmission of data. With respect to an IEEE 1394 network, the invention is concerned only with the acquisition of the PHY transmission speeds within the IEEE 1394 network, and for this reason the link speeds of the respective nodes are not considered further. For a different acyclical network in which the PHY transmission speeds are identical to the actual transmission speeds, because apart from the PHY speeds of the nodes involved no further speeds of speed determining components have to be taken into account, the invention can also be used to acquire the actual transmission speeds. In what follows, the term transmission speed will be used in the sense of the maximum actual transmission speed, and the term PHY transmission speed will be used for the maximum physical data transmission speed for transmissions of data between network stations.
In an IEEE 1394 bus network, both asynchronous communications links and isochronous communications links can be set up between nodes. In order to ensure the highest possible data transmission rates on the network, it is advantageous to know the transmission speeds between the individual nodes of the network. If a communication is carried out between network nodes at the minimum supported speed, the speed S100, even though the nodes which are involved in the data exchange can carry out the exchange of data at a higher speed than the minimum network speed, bandwidth of the data bus is added. If, on the other hand, an attempt is made to set up a communication at a speed which is higher than the PHY speed of one of the nodes involved in the exchange of data, or higher than the link speed of the starting node or higher than the link speed of a destination node, this attempt fails since data cannot be passed on or transmitted or received in this one involved node. In order, therefore, to be able to define an optimum transmission speed for any exchange of data in the network, it is consequently necessary to know the transmission speeds between all the network nodes present in the network, which presumes knowledge of the respective PHY transmission speeds.
An acyclical network has a hierarchy. Any network node has a maximum of one parent node and no, one or a plurality of children nodes with which it is directly connected. These directly neighboring nodes are referred to as adjacent nodes. The PHY transmission speed between two neighboring nodes is obtained from the minimum value of the PHY speeds of the two adjacent nodes, or is unknown if one of the two PHY speeds is unknown.
If the structure of the network changes, the transmission speeds between the nodes of the network have to be determined afresh. In the case of a network which is embodied as an IEEE 1394 network, a reset operation (bus reset) is carried out on the data bus, in particular if an electric/electronic device is connected to the data bus or an electric/electronic device is disconnected from the data bus. In the case of a reset operation on the data bus, each IEEE 1394 node which is connected to the data bus transmits a self-ID information item (ID identification) to the other nodes. This ensures that every node in the network is informed of which other nodes are connected on the network. By reference to the self-ID information, which is transmitted with so-called self-ID packets, it is possible to determine a bus topology of the network bus. In particular, a uniquely defined integral ID code number can be assigned to each node with the aid of the self-ID packets. Which nodes are adjacent in the bus topology results from port status fields of the self-ID packets and from the knowledge of how the ID code numbers on an IEEE 1394 bus are assigned after a reset operation according to the port statuses of all the nodes on the bus. A person skilled in the art is familiar with the procedure for assigning the ID code numbers so that at this point a detailed presentation will not be given. In addition, the self-ID packets contain information about the PHY speed of the respective node.
U.S. Pat. No. 5,504,757 discloses a method for determining a transmission speed between any two nodes of an IEEE 1394 network in which a common preceding node is determined for a starting node and a destination node, and in each case the transmission speed from the starting node to the common preceding node and from the destination node to the common preceding node is determined, and the transmission speed from the starting node to the destination node is then set to be equal to the minimum value of the transmission speed from the output node to the common preceding node, and to the speed from the destination node to the common preceding node. The known method does not acquire the actual transmission speeds including the link speeds, but rather only the PHY transmission speeds. In the proposed method, the transmission speed from the starting node to the destination node is determined in each case directly before a communications link is set up.
If communications links are frequently implemented in succession between the same nodes of the network without a bus reset operation having taken place in the meantime, the method steps for determining the maximum transmission speed are carried out repeatedly, which amounts to wasting computing power.